They generated some of the most powerful shaking in nearly 20 years in a region with a long history of seismic activity, and the biggest temblor — registering a magnitude of 7.2 — was twice as powerful as the January earthquake in Haiti.

Yet all that geologic disturbance caused relatively little damage, particularly north of the border.

The explanation is a matter of fortunate geography and soil conditions, factors that are likely to significantly determine which areas of San Diego County fare better or worse in future quakes.

The epicenter of the 7.2 temblor was in a rural and sparsely inhabited area surrounding the village of Guadalupe Victoria. The closest major city — Mexicali, with a greater metropolitan population of nearly 1 million — is 38 miles to the north, far away enough that much of the quake’s energy dissipated before reaching there.

“We were lucky the epicenter was in an agricultural region,” said Morgan Page, a geophysicist with the United States Geological Survey. “The quake was strong and shallow, about six miles below the surface. If it had hit closer to a city, it would have been pretty bad.”

There were reports of structural damage in some parts of Mexicali, resulting in the deaths of at least two people.

While distance helped protect the city, its underlying soil did not. Mexicali is atop a sedimentary basin of soft, loose soil that is not only more vulnerable to shaking, but also amplifies the movement produced by an earthquake.

“The soil liquefies. It behaves like a bowl of jelly,” said Debi Kilb, a seismologist at the Scripps Institution of Oceanography in La Jolla, which is part of the University of California San Diego.

San Diegans are not immune to this phenomenon.

In areas developed on top of looser sediments, such as around Mission Bay and in Mission Valley, the underlying soils are at risk of liquefaction, said Tom Rockwell, a professor of geology at San Diego State University.

During Sunday’s quakes, he said, residents in those areas would have felt the shaking more strongly than in, say, La Mesa, where the subsurface tends to consist of a granite bedrock.

The temblors were felt differently in Mexico than in the United States as well. Residents of Mexicali and those living closer to the epicenter experienced sharper movements than people farther away.

“It would have been quite jolting in Mexicali,” said Steven M. Day, a professor of seismology at SDSU.

The violent, shorter-frequency waves typically weaken first, leaving longer, lower-frequency waves to roll through great distances. On Sunday, the longer waves traveled north along a mountain range that carried and sustained the quakes’ energy.

“That’s what we felt in San Diego, that rolling sensation created by the longer energy waves,” said Pat Abbott, a professor emeritus of geology at San Diego State. “The fact that there was perceptible shaking that went on for as long as it did suggests this really was a major quake.”

Or perhaps two earthquakes, said Rockwell, who left ﻿with colleagues yesterday to inspect the 43-mile Laguna Salada fault zone southwest of Mexicali. Geologists believe that fault was the source of the largest quakes Sunday.

Rockwell said early data suggest the 7.2 temblor may have actually been a pair of smaller quakes happening in quick succession, combining their power.

He and other researchers will look for visible, measurable signs of ruptured earth, including newly shifted ground or offset roads and fences.

Another aspect that will be studied is how many temblors occurred, said Peter Shearer, a professor of geophysics at UCSD. For several days before the 7.2 quake, the area was shaken by a series of smaller earthquakes in the magnitude 3 and 4 ranges. There have been dozens of aftershocks as well, including at least seven exceeding magnitude 5.

“This region tends to have swarms of quakes,” Shearer said. “People will look to see if there’s … anything distinctive that could have provided a clue that a larger quake was coming.”

Whatever the revelations, the earthquakes are part of larger story that’s been going on for at least 8 million years. The recent quakes happened in a transition zone between two tectonic systems.

To the north in California, the Pacific and North American plates are sliding past each other, divided by the San Andreas fault zone, at a speed of about two inches per year.

South of the border, tectonic activity is more complicated. Not only are the Pacific and North American plates grinding against one another, but the sea floor beneath the Gulf of California also is splitting apart.

The result is similar to a zipper being pulled open, with the Baja California peninsula on the Pacific plate slowly pivoting out from the Mexican mainland and the gulf becoming bigger.